The mechanism of the interaction of thrombin with platelets will be studied. Emphasis will be on further characterizing the specific action of chymotrypsin, which has been shown to cause a change in the rate, but not the extent, of thrombin induction of platelet aggregation and secretion. Gel electrophoresis of platelets labeled on their exterior surface with radioactive markers will be used in an attempt to define which surface proteins are essential for the thrombin reaction and which are modified by the action of thrombin.